1. Field of the Invention
This invention relates to a method and apparatus for measuring the movement of a spiral wound wire rope during well operations using a non-contacting means in order to determine the depth in a well of equipment supported by the wire rope. The invention may be employed to measure movement of wire rope in other applications, such as in elevators or with cages in mine shafts that are supported by wire rope.
2. Prior Art
The accurate measurement of equipment depth in a well is of major importance, and various arrangements have been employed to effect such measurement.
Some arrangements sense the rotation of the crown-wheel on the derrick and thereby provide an indication of the longitudinal movement of the wire rope which actuates the kelly and the drill stem. Such arrangements require installation and maintenance work at the top of the derrick which may be dangerous, especially in bad weather or on floating drill rigs. Such arrangements suffer the disadvantage of inaccuracies introduced by slippage of the wire rope on the crownwheel or by slippage of a mechanical wheel employed to sense movement of the crownwheel.
Another arrangement measures the rotation of the drum of the drawworks and the layers of the wire rope that are spooled on the drum to measure longitudinal movement of the wire rope. Such an arrangement is rather complex, and it is not convenient for use in well logging because well logging techniques require self contained apparatus that can be readily applied to a drilling rig and that can be moved from one drilling rig to another.
Still another arrangement measures the incremental movement of the kelly by apparatus mounted at the top of the kelly. Such an arrangement is subject to considerable vibration and shock due to the movements of the kelly. It is difficult to service particularly during the drilling operation.